1. Field of the Invention
This invention relates to a rapid process for substantially increasing the molecular weight and viscosity of polymers of caprolactam during melt processing. More specifically, this invention relates to a process in which the increase in the viscosity is effectuated by the addition of an effective amount of a narrow class of phosphite compounds. Polymeric compositions prepared in accordance with the process of this invention are capable of being fabricated into useful shaped articles of manufacture, e.g. filaments, both mono- and multifilaments, films, tapes, ribbons, rods, laminates, etc.
2. Description of the Prior Art
Polymers of caprolactam are disclosed in the prior art as having many and varied uses. For example, these polymers can be formed into filaments in which the polymer chains are oriented along the filament axis. Such filaments have many uses in commercial applications, as for example, in the production of fibers for tire cord, textiles and the like. Similarly, these polymers can be fabricated into other useful shaped articles, as for example, gears, lawn mower housings, skate boards and the like.
The melt index of polymers of caprolactam provides an indication of the viscosity of the polymers and of their molecular weights, i.e. chain length, in that these parameters are inversely related. In general, polymers having low melt indexes are desirable because of the improved properties of fibers and other shaped articles made therefrom. For example, fibers made from polymers of caprolactam having relatively low melt indexes have increased tensile strength, durability and impact resistance.
Several processes have been proposed in the prior art for increasing the viscosity and molecular weight of polyamides, such as poly(hexamethyleneadipamide). For example, U.S. Pat. No. 3,763,113 discloses a method for increasing the molecular weight of polyamides having recurring -CONH-alkylene-NHCO-alkylene- units, such as poly(hexamethyleneadipamide), by treating the polyamide with a phosphoric acid derivative in the presence of an inert gas, such as nitrogen. Similarly, U.S. Pat. Nos. 3,551,548 and 3,763,113, each broadly describes a method for increasing the relative viscosity of polyamides generally, and poly(hexylamethylene adipamide) specifically, by sweeping a molten mixture of the polyamide and a phosphorous compound with an inert gas.
Each of these processes provides various adverse effects. For example, in each process an inert gas is employed, which in high concentrations causes uneven finishing, i.e. variations in the degree of polymerization throughout the polyamide, which results in a nonuniform polymer. Furthermore, inert gases are expensive, and require additional equipment and monitoring which can increase the cost of commercialization of the process, even though the inert gas is used in small amounts.
A further defect in these processes is that broadly speaking it is not readily apparent that they can be employed with polyamides of the type exemplified by polycaprolactam. For example, U.S. Pat. No. 3,763,113 clearly restricts the use of the process of that patent to polyhexamethylene adipamide type polyamides. While the disclosures of U.S. Pat. Nos. 3,551,548 and 3,763,113 are not so restrictive, as will be shown hereinbelow the phosphorous compounds described as useful in the processes of those patents cannot be used with polyamides of the type epitomized by polycaprolactam (Nylon 6), polyundecanolactam (Nylon 11) and polylauryllactam (Nylon 12).